This paper presents a nonlinear finite element analysts of substandard reinforced concrete beam-column joint strengthened by cast in-situ expansion The nonlinear finite clement analysis is based on two-dimensional reinforced concrete planar elements to simulate beam, column, joint panel and joint expansions. The one dimensional discrete joint element is employed to simulate local deformation at the interface between members and expansions Five specimens, control specimen and four strengthened specimens have been analyzed. The analysis correctly reproduces load-drill ratio relations Including hysteretic loops, deformed shape, cracking process and failure mode The load resistant mechanism of the strengthened specimens is also investigated The strain profile of steel beam bar is examined to understand the transmission of forces in expanded joints. The principal compressive stress is analytically computed in the joint panel zone and expansion parts. The analysis illustrates the diagonal strut as the main load resistance in the joint panel. For strengthened specimens, the plot of analytical principal stress identifies two load bearing mechanisms In addition to the main diagonal strut in the joint parel, the inclined strut mechanism also forms along the edge of the expansions. The size of expansion has a direct impact on the capacity of this inclined strut. The finite element analysis is also applied to exterior beam-column joints. Similar conclusions as the interior beam-column joint can be drawn.